1. Technical Field of the Invention
The present invention relates to a laminated disc such as a DVD (Digital Video Disc) and a turntable on which laminated disc is placed for reproduction of information.
2. Description of Prior Art
FIG. 20 is a cross-sectional view of a prior art turntable which is disclosed in Japanese Patent Preliminary Publication No. 8-7425. A turntable 102 has a hub 105 into which a motor shaft 104 is press-fitted or securely bonded to drive the turntable 102 in rotation. A disc 101 is placed on the turntable 102 and clamped by a clamper 103 so that the disc 101 is firmly held between the turntable 102 and the clamper 103 in a sandwiched relation. The hub 105 fits into a center hole 106 in the disc 101 with a very small clearance, thereby concentrically positioning the disc 101 with respect to the rotational axis of the motor shaft 104. The disc 101 is rotated by a driving force of a spindle motor 100.
FIG. 21 is a perspective view of a conventional disc 101 on which information is recorded. The disc 101 has a center hole 106 in its center and placed on the turntable 102.
A DVD is a laminated disc which includes two discs bonded together back-to-back. When the two discs are bonded together, they are placed one over the other slightly eccentric due to alignment error. This alignment error varies within a predetermined tolerance and results in a small step in the wall of the center hole of the discs. When a DVD is loaded onto the aforementioned conventional turntable, the stepped wall of the center hole of the disc may be caught by the hub 105 so that the disc 101 will not descend to the supporting surface of the turntable.
A DVD is usually loaded onto the positioning hub of the turntable so that the hub extends through the center hole with a very small clearance between the DVD. Burrs projecting from the the bonded interface of the disc into the center hole also cause a problem similar to that caused by the stepped wall or may present a problem that the disc is not concentric with the positioning hub. A further problem with the conventional disc is that the bonding agent spreads from between the two discs into the center hole during the manufacture of the disc or during operation of the turntable at elevated temperatures.